I started a few perennials from seed:

Purple Coneflower, Delphinium, Lupine, Oriental Poppy, Lavendar and am trying to propegate hydrangea also right now. I have a couple of trays and I am starting to notice that with one tray, the seedlings are growing ok, (overcrowded right now and need to be transplanted asap, but growing ok) and with the other tray the soil has turned greenish (like a skum on the top)...how do I get rid of it and what is it?

Also, when can I transplant into the soil outside (what height and tempurature outside does it need to be?

THANKS AS ALWAYS